1. Field of the Invention
The present invention relates to arrangements for driving a member in bidirectional fashion along a linear path, and more particularly to arrangements for reciprocating a shuttle assembly adjacent print paper in a line printer.
2. History of the Prior Art
It is known to provide a line printer in which a shuttle assembly including a hammer bank is driven in reciprocating, bidirectional fashion along a linear path adjacent a platen-supported ribbon and print paper or other printable medium as the individual hammers of the hammer bank are actuated so as to impact the printable medium and effect the desired printing. An example of such an arrangement is provided by U.S. Pat. No. 3,941,051 of Barrus et al, issued Mar. 2, 1976 and commonly assigned with the present application. The arrangement shown in the Barrus et al patent drives the shuttle assembly using a counterbalanced, cam controlled positive drive mechanism.
In the driving arrangement shown in the Barrus et al patent, the controlling cam surfaces must be precisely generated for the desired essentially trapezoidal velocity function, although substantial wear can have an adverse effect on the nature of the motion. With such an arrangement, a large drive motor and flywheel are desirable for velocity stability, and there are practical limitations on the shuttle rate that can be achieved.
An alternative arrangement which avoids some of the problems present in the system of the Barrus et al patent and which provides certain other advantages is shown in a co-pending application of Jerry Matula, Ser. No. 765,873, filed Feb. 4, 1977, now U.S. Pat. No. 4,180,766 issued Dec. 25, 1979, and commonly assigned with the present application. The printer disclosed in the Matula patent drives the shuttle assembly using a linear motor. The linear motor includes a coil coupled for linear movement in conjunction with the shuttle assembly and a surrounding permanent magnet. The coil is bidirectionally energized by a circuit which is sensitive to movement of the shuttle assembly between opposite limits and which energizes the coil in accordance with the difference between the actual and the desired velocity of the shuttle assembly. The coil energizing circuit saturates whenever the actual velocity of the shuttle assembly falls below a minimum value to provide a large driving current to the coil following reversals in direction and at any other time that high energization of the coil may be needed. For the most part, however, resilient stop elements provide substantial rebounding force on change in direction so that servo control may be employed to provide the small amount of energizing current necessary to maintain the shuttle assembly at a nominal velocity. Other examples of linear motor arrangements used in printers and other peripheral equipment are provided by U.S. Pat. No. 4,149,808 of Matthias et al issued Apr. 17, 1979 and U.S. Pat. No. 4,151,447 of von der Heide et al, issued Apr. 24, 1979.
An alternative arrangement for bidirectionally driving a shuttle assembly is shown in a co-pending application of Jerry Matula et al, Ser. No. 7,789, filed Jan. 30, 1979, now U.S. Pat. No. 4,239,403 issued Dec. 16, 1980, and commonly assigned with the present application. The arrangement shown in the Matula et al patent substantially reduces or eliminates shaking and other vibratory motion which can result from reciprocating movement by using a counterbalance arrangement in which an elongated counterbalancing bar extending between an opposite pair of rotatable pulleys on one side of the pulleys counterbalances a hammer bank shuttle assembly mounting member of similar mass extending between the opposite pair of rotatable pulleys on the opposite side thereof from the counterbalancing bar. The shuttle assembly mounting element and the counterbalancing bar which are attached to a band formed into an endless loop encircling the opposite rotatable pulleys are held in position against the pulleys by a magnet assembly extending between the mounting element and the counterbalancing bar in the region between the opposite rotatable pulleys. The mounting element and included hammer bank shuttle assembly are bidirectionally driven between opposite limits of movement defined by engagement of a block on the outside of the counterbalancing bar with either of an opposite pair of springs by a DC motor coupled through a belt and pulley arrangement to one of the rotatable pulleys. The DC motor is bidirectionally driven by a current having a polarity which reverses with the opposite reversals in direction of the shuttle assembly and which is momentarily of large value such as during reversals in the shuttle assembly and otherwise of a relatively small value required to servo the shuttle assembly at a desired nominal velocity.
The arrangement shown in Matula et al U.S. Pat. No. 4,239,403 is advantageous in a number of respects, and particularly with respect to the substantial reduction or elimination of vibration and other undesired motion due to the counterbalancing of the arrangement. However, there may be instances where alternative arrangements for driving the counterbalanced mechanism are desired. The belt and pulley couplings between the DC drive motor and the driven one of the opposite rotatable pulleys can introduce substantial friction. The DC motor itself can introduce substantial friction. Moreover, there can be significant thrust loads on the bearings of the motor and the bearings of the shaft mounting the driven pulley because of belt tension. A still further problem may arise in connection with commutation of the motor. Because of the relatively short rotational stroke of the motor between opposite limits of movement of the counterbalanced arrangement, the brush commutator of the motor may wear excessively rapidly in certain applications.
Accordingly, it would be desirable to provide an alternative arrangement for bidirectionally driving a counterbalanced hammer bank shuttle drive. It would furthermore be desirable to provide a driving arrangement which is relatively simple in design and which shares parts with an arrangement for magnetically holding the shuttle assembly mounting element and the counterbalancing bar against the pulleys. It would still furthermore be advantageous to provide a driving arrangement which is of relatively low friction and which is capable of maintaining the hammer bank shuttle assembly at a desired nominal velocity using a minimum of drive current.